Positron annihilation lifetime study of organic-inorganic hybrid materials prepared by irradiation N. Djourelov a, * , T. Suzuki a , M. Misheva b , F.M.A. Margac ¸a c , I.M. Miranda Salvado d a High Energy Accelerator Research Organization (KEK), 1-1, Oho, Tsukuba, Ibaraki 305-0801, Japan b Faculty of Physics, Sofia University, 5 J. Bourchier Blvd., 1126 Sofia, Bulgaria c Physics Dept., Nuclear and Technological Institute, E.N. 10, 2686-953 Sacave ´m, Portugal d Glass and Ceramics Engineering Dept., UIMC, University of Aveiro, 3810 Aveiro, Portugal Received 22 March 2004 Available online 21 December 2004 Abstract Silica based hybrid materials, some containing zirconia, prepared by gamma-irradiation, were studied by positron annihilation lifetime (PAL) spectroscopy, in the temperature range of 30–370 K. One long-lived component was observed in group of samples without zirconia. The behavior of this component as a function of temperature resembles that for polymers. This is explained by the elastomer like structure of the samples and the excellent linking between the inorganic and organic networks. The average radius, R, of the free-volume holes was in the range 0.26–0.42 nm depending on the temperature. In the group of samples containing zirconia two long-lived components were observed. A model with three different free-volume holes was suggested to explain the PAL results. Small closed holes (R  0.26 nm) and large closed holes introduced by Zr (R  0.5 nm), the sizes of both holes not changing with the temperature, together with intermediate polymer-like free-volume holes – the same behavior as those present in samples not con- taining zirconia. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Hybrid organic-inorganic materials are organically modified inorganic oxide networks that can be prepared by mixing an alkoxide, like tetraethylortosilicate (TEOS) with an organosilane polymer such as poly- dimethilsiloxane (PDMS). In appropriate processing conditions, the mixture gives rise to the chemical incor- poration of the organic species into the growing silica network [1]. The organic constituent is added to tailor the mechanical properties of the material: hybrid mate- rials have been produced with quite different properties (mechanical, thermal, optical, etc.) ranging from those characteristic of conventional glasses to those typical of flexible materials (elastomers). The resulting materials have a wide range of applications. Radiation can be used to promote polymer cross-linking [2] acting on the material properties. In the present work gamma irra- diation was used to prepare the hybrid samples, from irradiation of the homogeneous mixture of the precur- sors in absence of solvents and water. Positron annihilation lifetime (PAL) spectroscopy has been accepted as a sensitive nondestructive tool for studying pores and free-volume holes in solids [3– 5]. After entering condensed matter the positron is slo- wed down by various interactions and at the end of its track it annihilates either directly on an electron or via forming an intermediate state positronium (Ps). Ps is the bound state of a positron and an electron and ex- ists in two forms: singlet (para-positronium – p-Ps) and triplet (ortho-positronium – o-Ps). The intrinsic lifetimes of p-Ps and o-Ps in vacuum are 0.125 ns and 142 ns, 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.11.015 * Corresponding author. Tel.: +81 29 8796005; fax: +81 298 644051. E-mail address: nikdjour@post.kek.jp (N. Djourelov). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 351 (2005) 340–345